While the individual writer may be primarily responsible for historical
accuracy, it appears to be the case that many producers retain
consultants with specialized knowledge. In the case of military films,
that can often include former military personnel. George Scott's Patton,
for example, was based on Omar Bradley's biography, and Bradley was a
consultant.
There's an enjoyable book: "History According to Hollywood", which
concludes that, by and large, film makers have done well in re-creating
historical settings and costumes, but less well in depicting faithfully
historical events.
Peter Latham
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Online resources for film/TV studies may be found at ScreenSite
http://www.tcf.ua.edu/screensite